The invention relates to a monomode optical transmission fibre whose end portion has a tapered core and cladding, on which end portion a lens made of transparent material and having a spherical surface is arranged. The invention also relates to a method of manufacturing such a transmission fibre.
Such a transmission fibre and method are known from the article "Efficient Coupling Of Laser Diodes To Tapered Monomode Fibres With High-Index End" in Electronics Letters, vol. 19, no. 6, March 1983, pages 205-207. As described in this article, monomode transmission fibres with a tapered core and cladding, in comparison with transmission fibres with a straight and flat end portion, have substantially better properties as regards coupling efficiency, permissible offset of the fibre relative to a radiation source, which is for example a diode laser, and the feedback of radiation to the radiation source as a result of reflections inside the fibre.
The coupling efficiency is to be understood to mean the quotient of the radiation intensity received from the source by the transmission fibre and the total radiation intensity of this source.
As a result of the feedback effect a part of the radiation emitted by the diode laser can re-enter the diode laser. This feedback may give rise to an undesired modulation of the radiation emitted by the diode laser.
The coupling efficiency can be increased substantially by providing the tapered end portion of the monomode transmission fibre with a lens. As is described in the above mentioned article, this lens can be formed by immersing the tapered end portion of the fibre in a liquid transparent material, withdrawing the tapered end portion from the liquid material, and allowing the material which has adhered to the fibre end to solidify until a solid lens shape is formed. In said article it is stated that the refractice index of the lens material should be substantially higher than that of the core material, as a requirement to obtain an adequate numerical aperture for the combination of fibre end and lens. Owing to the large difference between the refractice index of the lens and that of the fibre core reflections at the lens-fibre interface may occur, which may give rise to feedback to the radiation source and a reduced coupling efficiency.